Measuring Geological Time

Dr. Ahmad Redaa

2025-05-03

Outlines

The Geological Time Scale

Source: International Commission on Stratigraphy (ICS)

Definition:
The geological time scale divides Earth’s history into intervals based on major events in the rock and fossil record.

Geological Time Scale: Structure and Milestones

Eons of Geological Time

The four eons of Earth’s history. Source: Steven, E. (2015)

Phanerozoic Eon

The eras (middle row) and periods (bottom row) of the Phanerozoic eon. Source: Steven, E. (2015)

Paleozoic Era (541–252 Ma)

Mesozoic Era (252–66 Ma)

The periods (middle row) and epochs (bottom row) of the Cenozoic era. Source: Steven, E. (2015)

Cenozoic Era (66 Ma–Present)

Relative Dating Methods

The Principle of Superposition

Source: https://www.geologyin.com/

Source: https://www.geologyin.com/

Inclusions

A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii. The lava flow took place some time after the diorite cooled, was uplifted, and then eroded. (geological rock hammer head for scale). Source: Steven, E. (2015)

The Principle of Cross-cutting Relationships

Superposition and cross-cutting relationships in Cretaceous Nanaimo Group rocks in Nanaimo, B.C. The coal seam is about 50 centimetres thick. The sequence of events is as follows: a) deposition of lower sandstone, b) faulting of lower sandstone, c) deposition of coal seam and d) deposition of upper sandstone. Source: Steven, E. (2015)

Source: https://geologybase.com/

Unconformities

The four types of unconformities: (a) a nonconformity between older non-sedimentary rock and sedimentary rock, (b) an angular unconformity, (c) a disconformity between layers of sedimentary rock, where the older rock has been eroded but not tilted, and (d) a paraconformity where there is a long period (typically millions of years) of non-deposition between two parallel layers. Source: Steven, E. (2015)

Dating Rocks Using Fossils

A summary of life on Earth during the late Proterozoic and the Phanerozoic. The top row shows geological eras, and the lower row shows the periods. Source: Steven, E. (2015)

Isotopic Dating Methods

Other Dating Methods

Source: https://earthsci.org

Source: https://www.ontariobeneathourfeet.com/

Source: https://divediscover.whoi.edu

Understanding Geological Time

Geological Time as a Calendar Year

Key Milestones:

Humans have existed for only a few seconds on the geological clock!